An organic field effect transistor using an organic semiconductor as a channel region (current passage) of a field effect transistor (FET) can be fabricated inexpensively by ink-coating techniques such as screen printing compared to a FET using an inorganic semiconductor. Further, there are advantages that the organic field effect transistor is flexible and can be easily formed in a large size. Therefore, techniques for a practical use of the organic field effect transistor have been actively developed in recent years. The organic field effect transistor is classified to two types, that is, one is a low molecular type organic transistor using pentacene and so on and the other is a polymer type organic transistor using polythiophene and so on. Since the low molecular type transistor can be fabricated by vacuum deposition etc., it is excellent in reproducibility of device properties. Therefore, it has been used for a drive circuit of a display. However, the polymer type transistor is better in view of flexibility. Hence, it has been demanded to enhance performance of the polymer type transistor.
An organic field effect transistor using an organic semiconductor material utilizes insulation capacities of the organic semiconductor material for the off-state of the transistor. Since a low molecular material such as pentacene is deposited under vacuum to form a film, the obtained thin film hardly includes impurities. Therefore, the insulating state of the thin film can be easily maintained. However, it is difficult to maintain the insulating state of a thin film made of a polymer material since it is easily changed to an oxidized state, which is a conductive state, due to catalyst residues of the polymerization process, impurity ions contaminated during a wet process for forming the thin film, oxygen and moisture in the atmosphere and so on. Therefore, most conventional organic field effect transistors using the polymer material are used as a normally on-state transistor due to impurities contained in the semiconductor layer. Also, in organic transistors using the polymer material, it is difficult to control reproducibility of device properties because of the reasons mentioned above.
As a method for solving these problems, for example, there has been proposed a method using a conjugated polymer which is resistant to oxidation. However, the method has been essentially unable to control adverse affects of impurities contained in a semiconductor layer. When these impurities are contained in the semiconductor layer, a decrease in insulation properties of the conjugated polymer due to the doped impurities causes a subthreshold property to deteriorate, and transistor properties are remarkably deteriorated by hysteresis appearance due to polarization of the impurity ions. As other methods for solving the problems, there have been conducted various countermeasures such as removal of catalyst residues (metal ions) from a conjugated polymer material, thorough removal of solvent from a polymer film, thorough washing of tools for making a polymer film, production of a polymer film under a deoxygenated and dehydrated atmosphere in a globebox. However, there are restrictions in these methods that special facilities are required or thorough operations are necessary to achieve effects. Although other various methods have been proposed regarding the organic field effect transistor using the conjugated polymer material (for example, see Patent Documents 1-6), the problems set forth above have not been solved.